Unit 3 – Earth s Interior

Earth’s Interior
Earthquakes
Volcanoes
What are Earthquakes?
• Seismology – the study of earthquakes
• Where do Earthquakes occur?
 Fault – a break in the Earth’s crust along which blocks
of the crust slide relative to one another
 Faults occur in many places, but they are especially
common near the edges of tectonic plates where they
form the boundaries along which the plates move.
What causes Earthquakes?
 As tectonic plates push, pull or scrape against each
other, stress builds up along faults near the plates edges
and causes stress on the rocks, which causes plates to
deform.
 Deformation – the change in the shape of rock in
response to stress.
 2 ways of rock deformation:
1. Plastic deformation – acts like clay (can mold
easy), but does not lead to earthquakes
2. Elastic deformation – acts like a rubber band,
and does lead to earthquakes
 Elastic Rebound – sudden return of elastically deformed
rock to its original shape
Are ALL Earthquakes the same?
Plate Motion Prominent Fault Earthquake
Type Characteristics
Transform – occurs Strike-slip fault Moderate, shallow
where two plates slip
past each other
Convergent – occurs Reverse fault Strong, deep
where two plates push
together
Divergent – occurs Normal fault Weak, shallow
where two plates pull
away from each other
How do Earthquakes travel?
• Seismic Wave – waves of energy that travel through
the Earth
• Body waves – seismic waves that travel through the
Earth’s interior
 2 types of Body Waves:
1. P waves or Primary Waves – travel through solids,
liquids, and gases; are the fastest seismic waves
2. S waves or Secondary Waves – 2nd fastest, but
cannot travel through the entire Earth, like P waves
• Surface Waves – move the ground much like ocean
waves move water particles; cause the most damage
during an earthquake
Locating Earthquakes
• Seismographs – instruments located at or near
the surface of the Earth that record seismic
waves
• Seismogram – a tracing of earthquake motion
created by a seismograph
• When did it happen?
 An earthquakes starts when rock slips suddenly
enough along a fault to create seismic waves.
 Seismologists find an earthquake’s start time by
comparing seismograms and noting the difference in
arrival times of P waves and S waves
Where did it happen?
• Epicenter – the point on the Earth’s surface directly
above an Earthquake’s starting point
• Focus – point inside the Earth where an earthquake
begins
• Most common method to finding an Earthquake’s
Epicenter: S-P time method.
• http://ve.ou.edu/weaver/eq_locating/locatin
g.html
Discoveries of Earth’s Interior
• 1. Moho – marks the boundary between the Earth’s
crust and mantle
• 2. Shadow Zone – an area on the Earth’s surface where
not direct seismic waves from a particular earthquake
can be detected
• 3. Solid Inner Core – made mostly of iron, with some
nickel, and smaller amounts of oxygen, silicon, and
sulfur
• Earthquake Hazard – measures how prone an area is to
experiencing earthquakes in the future.
• An area’s earthquake hazard level is determined by past
and present seismic activity.
• West Coast – has a very high earthquake hazard level
because it has a lot of seismic activity
• Gulf Coast and Midwest – have much lower earthquake
hazard levels because they do not have as much
seismic activity.
• http://earthquake.usgs.gov/recenteqs/
Modified Richter Scale
Magnitude Estimated Effects Energy (Multiply by
31.7 each unit
higher) (**Examples)
2.0 Can be detected only 1
by seismograph
3.0 Can be felt at 31.7
epicenter
4.0 Felt by most in the 1,004.89
area
5.0 Causes damage at 31.855.013
epicenter
6.0 Causes widespread 1,009,803.9
damage
7.0 Causes great, 32,010,784
widespread damage
Worldwide Earthquake Frequency
(Based on Observations Sine 1900)
Descriptor Magnitude Average Occurring
Annually
Great 8.0 and higher 1
Major 7.0 – 7.9 18
Strong 6.0 – 6.9 120
Moderate 5.0 – 5.9 800
Light 4.0 – 4.9 About 6,200
Minor 3.0 – 3.9 About 49,000
Very Minor 2.0 – 2.9 About 365,000
Earthquakes and Buildings
• Modern Technology used to design and construct buildings and bridges to help
withstand earthquakes.
 1. Mass Damper – a weight placed in the roof of a building. Motion sensors detect building
movement during an earthquake and send messages to a computer. The computer signals
controls in the roof to shift the mass damper to counteract the building’s movement.
 2. Cross-braces – are placed between floors and they counteract pressure that pushes and
pulls at the side of a building during an earthquake.
 3. Active Tendon System – sensors notify a computer that the building is moving, then the
computer activates devices to shift a large weight to counteract the movement.
 4. Flexible pipes – help prevent water and gas lines from breaking. They are better able to
twist and bend without breaking during an earthquake.
 5. Base isolators – act as shock absorbers during an earthquake. Absorb seismic waves,
preventing them from traveling through the building.
 http://videos.howstuffworks.com/hsw/9217-investigating-earthquakes-seismic-wave-
protection-video.htm
 http://videos.howstuffworks.com/hsw/9216-investigating-earthquakes-earths-moving-crust-
video.htm
Are you prepared for an Earthquake?
• Before the Shaking Starts:
 Safeguard your house against earthquakes
 Put heavier objects on lower shelves
 Reinforce your home’s structure
 Store nonperishable food, water, a fire extinguisher, a flashlight with
batteries, and a first aid kit
• When the Shaking Starts:
 1. If indoors – crouch or lie face down under a table or desk
 2. If outdoors – lie face down away from buildings, power lines, trees
and cover your head with your hands
 3. If in a car – stop the car and remain inside
• After the Shaking Stops:
 Remove yourself from immediate danger
• Volcano – a mountain that forms when molten rock,
called magma, is forced to the Earth’s surface
• Two types of Eruptions:
 1. Non Explosive Eruptions
 Relatively calm outpourings of lava
 Release a huge amount of molten rock
 2. Explosive Eruptions
 Clouds of hot debris and gases shoot out from the volcano
 Can blast millions of tons of solid rock
 http://www.cotf.edu/ete/modules/volcanoes/vtypesvolcan
1.html
• 3 basic features that volcanoes share:
 1. Lava/Magma – hot liquid material, driving force
 2. Vents – holes in Earth’s crust where magma rises through
 3. Magma Chamber – stores magma inside volcano
• Magma – the composition of the magma determines whether a volcanic
eruption is non explosive, explosive or somewhere in between
• Water – a volcano is more likely to erupt explosively, if its magma has a
high water content
• Silica
 Explosive eruption – the vents are plugged, causing a buildup of
pressure and a massive explosion
 Non explosive eruption – the vents are not plugged
• What erupts from a volcano?
Non explosive – produces mostly lava
Explosive – produces pyroclastic material –
consists of rock fragments created by
explosive volcanic eruptions
• 4 Types of Lava
 1. Blocky Lava – usually oozes from a volcano,
forming jumbled heaps of sharp edged chunks
 2. Pahoehoe (pah HOY hoy) – lava flows slowly, like
wax dripping from a candle, forming a glassy surface
with rounded wrinkles
 3. Aa (AH ah) – slightly stiffer lava that pours out
quickly and forms a brittle crust
 4. Pillow Lava – forms when lava erupts underwater
Blocky Lava
Pāhoehoe
A’a
Pillow Lava
• 4 Types of Pyroclastic Material
 1. Volcanic Blocks – largest pieces, consist of solid rock
blasted from the volcano
 2. Volcanic Bombs – large blobs of magma that harden in the
air
 3. Lapilli (luh PILL ee) – pebble like bites of magma that
become solid before they hit the ground
 4. Volcanic Ash – forms when the gases in stiff magma
expand rapidly and the walls of the gas bubbles explode into
tiny glasslike slivers
 http://www.geology.sdsu.edu/how_volcanoes_work/Tephra.html
 What are some major problems with these types of pyroclastic
material?
• Major Problems with Volcanic Ash:
 1. when mixed with water, has the same consistency as cement
 2. can collapse buildings, due to weight
 3. smothers crops
 4. dams up rivers valley’s
• As the ash and gases spread around the globe, they can
block out enough sunlight to cause the average global
surface temperature to drop noticeable.
 EX. – Eruption of Mt. Pinatubo, in the Philippines caused an
average global temperature to drop about 32.9* in 1991.
• 3 Basic Types of Volcanoes:
 1. Shield Volcanoes – built out of layers of lava from repeated
non explosive eruptions
 EX. – Hawaii’s, Mauna Kea – tallest mountain on Earth, measured
from the sea floor
 2. Cinder Cone Volcanoes – small volcanic cones made of
pyroclastic material
 EX. – Paricutin, in Mexico
 3. Composite Volcanoes -- referred to as stratovolcanoes; form
by explosive eruptions of pyroclastic material followed by quieter
outpourings of lava
 EX. – Mt. Fuji, in Japan
• http://pubs.usgs.gov/gip/volc/types.html
• The Formation of Magma:
 Volcanoes form by the eruption of lava and pyroclastic material
onto the Earth’s surface
 Volcanoes begin when magma collects in the deeper regions of
the Earth’s crust and in the uppermost layers of the mantle, the
zone of intensely hot and pliable rock beneath the Earth’s crust
 2 Factors the help form Magma:
 1. Pressure and Temperature – as pressure decreases and
temp. increases magma is formed
 2. Density – once formed, magma rises toward the surface of
the Earth because it is less dense than the surrounding rock
• Where do volcanoes form?
Tectonic plate boundaries are likely places for
volcanoes to form. The “Ring of Fire”
contains nearly 75% of the world’s active
volcanoes on land.
http://www.crystalinks.com/rof.html
http://www.acleanerenvironment.com/platetec
tonicnotes.html